Properties of Light and Visual Function

1. Dr. Mohammad Shehadeh Properties of Light and Visual Function

2. Light: may be defined as energy to which the human eye is sensitive Electromagnetic Spectrum:

3. Optical Radiation • Optical radiation lies between X-rays and microwaves in the electromagnetic spectrum • subdivided into seven wavebands. • Each of these seven wavebands group together wavelengths which elicit similar biological reactions

4. These seven domains are: • ultraviolet C (UV-C), 200–280 nm • ultraviolet B (UV-B), 280–315 nm; • ultraviolet A (UV-A), 315–400 nm; • visible radiation, 400–780 nm; • infrared A (IRA), 780–1400 nm; • infrared B (IRB), 1400–3000 nm; • infrared C (IRC), 3000–10000 nm.

5. As with all electromagnetic radiation, the shorter the wavelength, the greater the energy of the individual quanta, or photons, of optical radiation

6. Penetration through the ocular structures • The cornea and sclera of the eye absorb essentially all the incident optical radiation at very short wavelengths in the ultraviolet (UV-B and UV-C) and long wavelengths in the infrared (IR-B and IR-C). • UV-A is strongly absorbed by the crystalline lens • wavelengths in the range 400–1400 nm (visible light and near infrared), pass through the ocular media to fall on the retina.

7. The visible wavelengths stimulate the retinal photoreceptors giving the sensation of light while the near infrared may give rise to thermal effects • Because the refractive surfaces of the eye focus the incident infrared radiation on the retina, it can cause retinal damage, e.g. eclipse burns. (IRA)

8. Eclipse burn

9. Colour Vision • The visible wavelengths of the electromagnetic spectrum are between 400 nm and 780 nm. • The colour of any object is determined by the wavelengths emitted or reflected from the surface. • White light is a mixture of wavelengths of the visible spectrum.

10. Colour is perceived by three populations of cone photoreceptors in the retina which are sensitive to light of: • short wavelength (blue), • middle wavelength (green), • or long wavelength (red)

11. A congenital colour vision defect occurs if a cone pigment is absent or if there is a shift in its spectral sensitivity • Types of color vision defects ( dyschromatopsia ) • deuteranopia, indicate absence of green cone function, • Protanopia: , indicate absence of red cone function • Tritanopia : , indicate absence of blue cone function

12. The followings indicate a shift in the corresponding cone Sensitivity • deuteranomaly, (D-J) • protanomaly ( R-P ) • tritanomaly ( T- B )

13. Genetics for color vision • The X-chromosome carries genes encoding for red and green pigment  Of men 8% and of women 0.5% have a defect of the red/green system • the commonest is deuteranomaly which occurs in 5% of men and 0.3% of women • chromosome 7 carries the blue pigment gene: Tritan defects are rare

14. Color vision and ocular diseases • acquired optic nerve disease tends to cause red–green defects • An exception occurs in glaucoma : blue–yellow deficit • autosomal dominant optic neuropathy which initially cause a predominantly blue–yellow deficit • Acquired retinal disease tends to cause blue–yellow defects